PNNL technology probes the depths of radioactive waste

Results:

Scientists at PNNL are collaborating with Hanford Site contractors in eastern Washington state to deploy—for the first time—telescopic Raman spectroscopy to remotely identify mineral and chemical compounds in radioactive waste left at the bottom of waste storage tanks after liquid waste has been pumped out. Nuclear waste stored at Hanford was mixed together from a variety of radionuclide separation processes that date back to the 1940s, making the waste highly variable in content. Monitoring the many types of chemically complex radioactive waste found at Hanford is challenging, expensive and potentially hazardous to workers and the environment.

Why It Matters:

Determining the composition of residual waste—called "hardpan"—at the bottom of waste storage tanks allows scientists to identify and study the material to determine the best approach for removing it. Raman spectroscopy provides information on molecular structure, which can be used to identify minerals and chemicals present in samples. It also eliminates the need for waste to be sent offsite for expensive and time-consuming laboratory testing, offering the potential to save millions of dollars in sampling analysis costs. Safety is enhanced because Raman spectroscopy is controlled remotely, thus eliminating the need for workers to handle radioactive waste samples.

Method:

Scientists place the telescopic Raman probe inside a protective canister that is mounted on the surface of a large underground tank. The instrumentation and protective canister can be easily disassembled and moved to the next tank. The top of the tank rests about 8 feet below ground level, which allows controlled access to the tank via a pipe (riser) connected from the surface into the tank for monitoring purposes. The canister, which protects the telescopic Raman probe and surrounding environment from waste contamination, has optical windows that allow the telescopic probe to shine its laser beam directly through the riser and onto the bottom surface of the waste tank. The laser emits a high-intensity light beam that can be finely focused on the hard-pan waste. The laser beam transmits waste measurements back up to the probe, which is connected to the spectrometer through a fiber-optic cable.

Schematic of a telescopic Raman system mounted on
a riser from an underground tank.

The major difference between telescopic Raman technology and standard spectroscopic equipment is that the probe can be inserted into the tank as far as 300 feet away from the spectrometer. The distance protects both the spectrometer and workers from possible radiation exposure. The spectrometer, which is operated by scientists, interprets the waste measurement data and provides the exact concentrations of identified elements on a graph.

What’s Next:

In addition to providing scientists with valuable characterization data in support of various waste processing operations at the Hanford Tank Waste Treatment and Immobilization Plant, the Raman spectroscopic technology has broad applications for monitoring and characterizing stored radioactive waste across the U.S. Department of Energy's government sites and commercial industry.

Acknowledgements:

Funding is provided by the U.S. Department of Energy Office of Environmental Management.